Explanation:

- 1929: Bhagat Singh and Batukeshwar Dutt threw a bomb in the Central Legislative Assembly on April 8, 1929. This act was intended to make a loud statement against oppressive policies and to create awareness among the masses without harming anyone.

- 1928: This is incorrect. Bhagat Singh was involved in revolutionary activities during this time, but the assembly bombing was yet to happen.

- 1930: Incorrect year. The incident had already taken place by this time.

- 1931: Incorrect option. By this year, Bhagat Singh and his associates were facing trials.

1929 is the correct answer.